In certain types of heating, ventilating and air conditioning (HVAC) equipment, electric heating elements are incorporated in the equipment in combination with a motor driven blower and, possibly, a cooling type heat exchanger, such as an evaporator coil for a vapor compression cooling circuit or heat pump circuit. One problem associated with utilizing electric heating elements in an air conditioning system of the general type mentioned herein is the requirement to provide for positive shut-off of the electric heating elements if the system blower or air circulation fan motor is operating in a range of operating conditions which will result in hazardous heat buildup. For example, if the blower or circuiting fan motor is operating at a relatively low speed, or has shut-off for any reason, unwanted and rapid heat buildup or overheating of the system may occur.
Moreover, regulatory requirements for air conditioning systems which utilize electric heating elements can result in extensive testing for various blower or air circulating fan motor operating conditions. However, if a system control can be provided which would block or interrupt power to the electric heating elements when the blower or circulating fan motor was operating outside of a predetermined range of operating conditions, regulatory testing requirements could be reduced, system reliability increased and the chance of a hazardous operating condition could be avoided. It is to these ends that the present invention has been developed.